Method of bleaching cellulose



Patented Oct. 29, 1940 UNITED STATES 2,219,432 METHOD OF BLEACHING CELLULOSE Emil Schellcr, Lorsbach-in-Tannus, Germany, assig'nor to Deutsche Gold and Silbcr Scheide- Anhalt, FranHort-on-the-Mahi, Germany,

corporation of Germany No Drawing. Application May 18, 1938, Serial 9 Claims.

This invention relates to a method for preparing cellulose products of superior quality which may be used for the. manufacture of paper, for the productionbi" highly viscous solutions, and

for other purposes in which cellulose of great purity and high mechanical strength is of value.

More particularly, it relates to a process forbleaching cellulose in the natural state, or in the form of chemical wood pulp such as that resulting a from kraft and sulfite paper pulp procedures.

The starting material for use in the improved process forming the subject matter of this invention may be a cellulosic product which has been so protected during preceding treatments that it contains the cellulose in less degraded form. It is possible in carrying out my process to employ so-called paper cellulose which has frequently been subjected to previous treatments includin cooking at a high temperature.

The process forming the subjectmatter of this invention involves a multistage treatment of the cellulosic material under certain conditions. This treatment results in the production of a bleached cellulosic product of high alpha-cellulose con- 25 tent. Moreover, the productis one which will retain its white color and will not tend to darken subsequently, either when in the form of pulp, or

when converted into the finished product. This superior whiteness retentivity is one of the outstanding improved characteristics of cellulosic products treated in accordance with my process.

The invention may be carried out by first treating the cellulosic material, such as previously cooked paper cellulose, with an alkali in order to increase its alpha-cellulose content to the desired percentage. This alkaline treatment may be carried out in accordance with known procedures and serves to increase the alpha-cellulose content to a relatively high value, generally at least 95%.

After the preliminary alkali treatment, the cellulosic material is subjected to treatment with elementary chlorine in order to chlorinate the lignin present as part or the cellulosic material. 'lhls may be done by treatment with g e us 5 chlorine or with chlorine water. Subsequent to the preliminary chlorination the cellulosic product is neutralized with caustic soda solution, which may conveniently be done by treating the pulp with the caustic solution at a gradually increased moderate temperature until a condition of distinct alkalinityis obtained.

The cellulosic product, freed to a large extent of its lignin by the chlorination treatment, is now subjected to a partial bleach with an alkali hypo- 6 chloiite. This may conveniently be done at a In Germany Mayzl, 193'! moderately elevated temperature, the pH of the solution being maintained slightly on the alkaline side. The cellulosic product is then subjected to a final bleach with an active oxygen yielding 5 compound in a weakly alkaline solution. It is then freed in the usual manner from the bleaching liquor and acidified.

Those steps following the preliminary alkaline treatment of the cellulosic material may all be advantageously carried out in the same apparatus. Ordinarily I prefer to use an ordinary bleaching hollander.

'According to an alternate method of proceeding in accordance with this invention the crude cellulose may be subjected to chlorination with elementary chlorine and thereafter to the action of an alkaline agent for increasing its alpha-cellulose content. In other words the pre-treatment step of the previously explained procedure may follow the step of subjecting the cellulosic material to the action of chlorine in gaseous form or in the form of chlorine water. The hypochlorite bleach step and the subsequent treatment with the peroxygen compound are carried out subsequently to the alkaline treatment step in this alternate procedure.

In order that all steps forming my improved process may be entirely clear, the following detailed description of a preferred method of proceeding in accordance with my invention is given. It should be understood that this method is illustrative and not restrictive, and that various changes might be made in the method disclosed which would still come within the purview of my invention.

The preliminary alkaline treatment is carried out by treating the cellulosic material, which maybe raw cellulose, chemical wood pulp such as sulilte pulp or kraft pulp, ground wood, or some other form of cellulosic material, with an aqueous solution of caustic soda. The Na-OH content of this caustic solution may be as high as approximately 10% of the dry weight of the cellulosic product. This treatment is carried out until there is no longer used up any appreciable quantity of the caustic soda present. The process may be carried out at elevated temperaturesas high as the boiling temperature, under which conditions the treatment may require 10 hours or longer. In order to accelerate the alkaline treatment the temperature may be raised above C., temperatures for example of C. being employed. A temperature of 140 C. corresponds to a pressure in excess of atmospheric of approximately 2.5 atmospheres and, of course, requires the use oi! pressure equipment.

In place of caustic soda other alkaline agents such as lime or mixtures such as soda lime may be employed. The heating may be carried out in ordinary spherical boilers directly heated by steam. It is preferably carried out, however, in boilers indirectly heated with steam, the bath being circulated in cases wherein the cellulosic product is maintained stationary. Vertical boilers equipped with perforated bottoms and an overflow tube may be utilized. In pressure treatment of this sort the alkaline boiling pre-treatment may be complete within 1 to 2 hours.

It is possible to secure a higher alpha-cellulose content and also a higher yield by treatment with the alkaline agents in the cold. Such an alkaline extraction can be carried out, for example, with a 10% caustic soda solution at 10 C. using approximately 100 parts of the caustic solution per 10 parts of cellulosic product treated. Under such conditions where there is a large quantity of alkaline agent with reference to the amount of cellulosic product present the extraction is generally completed within one-half hour.

After completion of the alkaline extraction step the cellulose is freed from the alkaline liquor by careful washing which may be done by first treating the cellulosic product with hot water and then with cold water. Washing is preferably carried out while agitating the goods, for example, while they are maintained in a bleaching hollander.

Chlorination of the material which has been preliminarily treated with an alkaline agent is also expediently carried out in the bleaching hollander. Either chlorine in gaseous form or chlorine water may be employed. The chlorine is preferably fed to the moving material at a moderate temperature, if possible, one not exceeding about 20 C., until no further chlorine is used up, even upon prolonged operation of the hollander. A moderate excess of chlorine favors complete chlorination of the lignin. During the step of treating the material with chlorine it is possible to avoid too great an increase in the free hydrochloric acid content by continuously washing.

The chlorinated material is now neutralized, for example, by treatment with caustic soda or lime until the liquor shows a distinct alkaline reaction. The temperature may be increased to approximately C. The cellulosic product is now carefully washed. Since the fibrous material has the'tendency to retain tenaciously residues of the brown colored liquor present, it is advisable to permit the product to stand for several hours after the first washing. 'I'he brown liquor will difiuse very readily from the fibrous material, thus permitting ready completion of the washing.

The subsequent partial bleach is carried out with the aid of a clear bleaching powder solution or by means of a sodium hypochlorite solution. It is most expediently carried out at a moderately elevated temperature, for example, one within the range 30 to 35 C. The liquor should be maintained weakly alkaline in reaction. It is desirable to add approximately 0.1 to 0.2 gram of caustic soda per liter of liquor or an equivalent amount of calcium hydroxide in the form of milk of lime to insure the maintenance of this alkalinity. The hypochlorite bleaching step is continued until the active chlorine content of the liquor with continued standing no longer decreases, or

decreases only slightly. This bleaching may require from 2 to 6 hours or longer depending upon the hardness of the cellulosic product used. It is then washed until a sample when tested for active chlorine will show substantially no active chlorine.

The cellulosic material is now treated with the peroxygen compound. To carry out this step solutions of hydrogen peroxide, sodium peroxide or some other suitable per compound may be employed. The alkalinities of such solutions are maintained lower than those 01 solutions of per compounds previously suggested for the treatment of cellulosic products. The alkalinity of the liquor should be regulated so that objectionable decomposition of the cellulose with consequent decreases in viscosity will be effectively prevented. The alkalinity of the bath should generally be so regulated that it corresponds to less than 0.2 gram of free caustic soda per liter and, preferably, at approximately 0.01 gram of caustic soda per liter. The alkaline condition should be maintained throughout the entire bleaching, the liquor being alkaline even' at the end thereof.

When these conditions as to alkalinity are maintained it is possible to carry out the step of bleaching the cellulosic products with peroxygen compounds in bleaching hollanders. Air may be allowed access to the contents of these hollanders without any important decrease inviscosity occurring.

It has been observed when using weakly alkaline solutions of per compounds that thepresence of stabilizing agents such as sodium silicate is unnecessary. Sodium silicate, which gives rise to detrimental silicic acid, is definitely disadvantageous during this step of my process. It may be pointed out that silicic acid when present produces an undesirable increase in the ash content of the cellulosic product.

The oxygen bleaching step is expediently carried out at a temperature 01' at least 40 (3., preferably one within the range to C. The

bleaching is continued only until the active oxygen content 01 the bath no longer decreases or decreases only to a very slight extent. Bleaching generally requires from 2 to 4 hours at a temperature of 55 C. The amount of bleaching agent used up is generally lessthan 0.5% of the original weight of the cellulosic product when sodium peroxide or hydrogen peroxide is utilized. In exceptional cases, where the cellulosic product is especially diflicult to bleach, it may be subjected to a brief after treatment with a weak solution of hypochlorite. This subsequent chlorine treatment should be carried out with hypochlorite of lime or sodium hypochlorite solution having an active chlorine content of from 0.05 to 0.1% based on the dry weight of the cellulose.

After completion of the step wherein the cellulosic product is bleached with the peroxygen compound the material is ireed from the bleaching liquor in the usual manner. This involves washing, acidifying and further washing. In order to improve the degree of whiteness of the cellulosic product small amounts of a soluble sulfite such as sodium sulfite or sodium bisulfite, or in some cases even suliurous acid, may beadded to the dilute acid present duringthe acidification step. During the washing step, after acidification, it is advantageous to have present a small amount of ammonia in the wash water. Unless a small amount of ammonia is added to the alkaline agent traces of acid may still be present on the fiber after the acidity of the wash water has disappeared. These small traces of acid may bring about injuries during the drying step and also cause considerable decrease in the viscosity of the solutions finally produced.

In special cases where the highest degree of whiteness is required it may be advisable to add to the oxygen bleaching bath an emulsifying agents such as Calgon, may be carried out simultaneously with the peroxide bleaching step. If desired, both wetting agent and softening agent or either one alone, may be added to the bleach bath either after completion of this bleaching step or toward the end thereof.

It should be understood that the illustrative details and procedures previously given in describing the preferred embodiment of my invention are to be considered as illustrative and not restrictive. Various changes may be made in the temperatures, times, amounts, percentages, and

specific ways of proceeding which will fall within the purview of my invention. The invention is therefore not to be restricted by the details described in setting forth its preferred embodiment, but is to be determined in accordance with the appended claims.

I claim:

1. A process for the preparation of bleached cellulosic products which comprises the following steps carried out in the specified order: subjecting a cellulosic product comprising substantially cellulose of a high degree of purity which has not been chemically changed to an appreciable extent to the action of a non-oxidizing alkaline agent of high caustic content in order to increase its alpha-cellulose content; bleaching said cellulosic product with active chlorine in acid solution; neutralizing said cellulosic product by subjecting it to the action of an alkaline agent; bleaching said cellulosic product by contacting it with an alkaline solution of hypochlorite; and then subjecting said cellulosic product to the action of an'active oxygen-yielding compound in weakly alkaline solution substantially free from silicates.

2. A process for the preparation of bleached cellulosic products which comprises subjecting a cellulosic product comprising cell ose oi a high degree of purity which has not been chemically changed to an appreciable extent to the following steps carried out inthe specified order: subjecting said cellulosic product to the action of a non-oxidizing alkaline agent of high caustic content for the purpose of increasing its alpha-cellulose content; bleaching said cellulosic product with active chlorine in acid solution; treating said cellulosic product with an alkaline agent in order to neutralize its acidity; subjecting-said cellulosic product to the action of a hypochlorite in alkaline solution for the purpose of still further bleaching said cellulosic product; applying to said cellulosic product an active oxygen-yielding compound in weakly alkaline solution for the purpose of still further bleaching said cellulosic product, said weakly alkaline solution of an 'active oxygen-yielding compound being substantially free from silicates; freeing said cellulosic product from the residual bleaching liquor; and acidifying said cellulosic product.

e 3. Method for preparing bleached cellulosic products wherein a cellulosic product comprising cellulose of a high degree of puritywhich has not been chemically changed to an appreciable extent is subjected to a process involving the following steps, carried out in the specified order: contacting said cellulosic product with free chlorine in the form of chlorine gas or chlorinewater; subjecting said cellulosic product to the action of a nonoxidizing alkaline agent of high caustic content for the purpose of increasing its alpha-cellulose content; subjecting said cellulosic product to the action of a hypochlorite in alkaline solution for the purpose of bleaching said cellulosic product still further; and finally treat!- ing said cellulosic product with an active oxygenyielding compound in weakly alkaline solution substantially free from silicates for the purpose of still further bleaching said product and for increasing its whiteness retentivity.

4. A process as defined in claim 2 wherein said cellulosic product is subjected to washing with water after the steps of contacting said cellulosic product with bleaching agents and prior to the next succeeding step of said process.

5. A process as defined in claim 2, wherein the alkalinity of the active oxygen-yielding solution during the final bleaching step, calculated as =NaOH, does not exceed 0.2 gram per liter of bleachingliquor during substantially the entire step wherein said cellulosic product is contacted with said active oxygen-yielding compound.

6. A process as defined in claim 2 wherein the alkalinity of the solution containing said active oxygen-yielding compound during the final bleaching step falls within the range 0.01 gram NaOH per liter to 0.2 gram NaOH per liter, said alkalinity being maintained within said range during substantially the entire final bleaching 1 step of said process.

7. A process as defined in claim 2 wherein the alkalinity of the solution containing said active oxygen-yielding bleaching compund during the final bleaching step of said process falls within the range 0.01 to 0.1 gram NaOH per liter of solutionduring substantially said entire step.

8. A process as defined in claim 2 wherein there is present in said bleaching solution containing said active oxygen-yielding compound during the final bleaching step an emulsifying agent.

9. A process as defined in claim 2 wherein there is present in salC bleaching solution containing said active oxygen-yielding compound during the final bleaching step a softening agent.

. EMIL scam-rm. 

